Yeah, I must be getting sleepy lol. I have your manual here, looked for the second slot, found none, and forgot it lol.
Well, like I said, all I got left is some kind of grounding issue during assembly, or something else you missed that we're assuming is done correctly, like the 4-pin/8-pin thing.
At this point I'd take it out of the case, hook it up, no front panel connectors attached, and start it by touching a screwdriver to the two StartSw pins. lets hope there's a screw or standoff rolling around in there, or some case/mobo problem that developed during your rebuild.